Generally, the presentable maximum brightness for every video display apparatus is predetermined and the corresponding presented contrast ratio is predetermined. As a result, conventional video display apparatuses have controlled brightness information of a screen by using various technologies to improve the perceived contrast ratio. However, there have been little effects in actual images because of several reasons.
According to one of the technologies, screen brightness is controlled by using an average value of video signals (hereinafter, average picture level (APL)). If this method is applied to a plasma display panel (PDP), the maximum brightness level of the screen is controlled by an auto-power limit (APL) circuit that uses the average picture level.
If such an auto-power limit (APL) control method is described briefly, an average picture level is set high, the maximum brightness level of the screen is lowered to prevent overload to PDP device in an overall bright image. That is, the overall bright image has a high APL and this means that the average brightness of the picture is high. As a result, the maximum brightness of the picture is controlled darker to adjust the power consumed by a PDP module to be below a predetermined level.
An absolute contrast ratio that is a ratio of a maximum brightness to a minimum brightness presentable by the video display apparatus is determined by characteristics of the corresponding video display apparatus. With development of video processing technology, has been under development a technology of improving perceivable brightness ratio by both analyzing an input video image information and adjusting a corresponding video signal. In recent, a compensation value of a picture generated based on an average value of the brightness information of the overall input image brightness has been adapted frequently.
FIG. 1 is a graph illustrating output according to an input of a conventional video signal, using a method in the brightness ratio is adjusted based on a result of measurement of average brightness of an input image signal. That is, in case an average value measured from the input video signal is a predetermined value or smaller, a brightness ratio of a dark area is improved. In case the average value measured from the input video signal is the predetermined value or larger, a brightness ratio of a dark area is improved.
However, according to such the conventional technology, an entire picture is determined only based on the average value of the brightness information on the entire screen input pixels. The perceived brightness ratio could not be improved a lot because there are actual pictures having little change of the average value of the entire picture even when video information has been changing continuously.
For example, in case of a picture having pixels in a highly bright area and in a very dark area in a similar ratio, a picture having only pixels of middle brightness would have an average brightness value and it results in a problem that the brightness ratio cannot be improved.
To overcome such a problem, brightness information on each pixel of a picture in an actual moving picture is analyzed and a relative perceived brightness ratio of the picture is adjusted according to the brightness distribution.
FIG. 2 is a graph illustrating a method of adjusting brightness according to brightness distribution in each conventional band. According to FIG. 2, picture brightness is divided into predetermined areas for bands, respectively, and corresponding brightness information of each band is determined to adjust the brightness ratio.